Electroless copper-plating (chemical copper deposition) is a method to coat surfaces with thin copper layers and this method has been used extensively in the art, especially when manufacturing printed boards. A typical method for electroless copper-plating comprises the preparation of the surface to be copper-plated so that it will be catalytically active, whereupon the surface is immersed in a bath containing an alkaline aqueous solution, having a pH of from 11.5 to 13.5, of a cupric salt, a complexing agent for cupric ions, a first reduction agent of aldehyde type and optionally a second reduction agent as well as the usual additives in order to improve the stability of the bath and to lower its surface tension.
The reactions occuring upon chemical copper deposition are very complex and are not completely investigated. In principle, the copper deposition functions as a redox reaction where the reduction agent is oxidized during electron transfer to the cupric ions which are reduced to metallic copper. When using the known methods for electroless copper-plating and the known baths, it is difficult to obtain a copper layer which is ductile in the holes at a speed which is sufficiently high to enable a rational production. The copper layer, which constitutes the connection between the two sides of the printed board and also is the base of the soldered joint with the components is exposed to a heat shock when the component is soldered and this shock can break the connection. In accordance with standard specifications, the layer must resist a temperature of 260.degree. C. for 10 seconds without having any fractures.
The methods and baths known up to now for chemical copper deposition produce deposited copper which is not sufficiently ductile at lower temperatures of the bath.